Overwrite individual tables in HDF5 file

Add the ability to overwrite a single table within an HDF5 file without
affecting other files if user passes both `append=True` and
`overwrite=True` to the `Table.write()` method. (Using `overwrite=True`
alone deletes the HDF5 file, and any other datasets inside it, while
`append=True` alone can only add new datasets to an HDF5 file.)
Adds `test_read_write_existing_append_overwrite` function to test this
combination of arguments to `write()`.

Closes #2624
Document append+overwrite arg combo for HDF5

See #2624